1. Field of the Invention
The present invention relates to a vehicle motion control device that causes a yawing moment on a vehicle for bringing the turning state of the vehicle into a target state by exerting braking force by brake fluid pressure on a predetermined wheel.
2. Description of the Prior Arts
There has conventionally been a demand for controlling a vehicle motion such that the turning state of the vehicle is brought into a target state (for example, into a neutral state), when the turning state of the vehicle is in an understeer state or oversteer state during the turning of the vehicle. The understeer state can be eliminated by exerting braking force by brake fluid pressure on a predetermined wheel for causing a yawing moment on the vehicle in the direction same as the turning direction of the vehicle. On the other hand, the oversteer state can be eliminated by exerting braking force by brake fluid pressure on a predetermined wheel for causing a yawing moment on the vehicle in the direction opposite to the turning direction of the vehicle.
On the other hand, a brake conduit for supplying brake fluid pressure to a wheel cylinder of each wheel in a four-wheel vehicle is generally composed of a dual circuit (a so-called X-conduit) comprising a circuit for supplying brake fluid pressure to each wheel cylinder of a front-right wheel and a rear-left wheel and a circuit for supplying brake fluid pressure to each wheel cylinder of a front-left wheel and a rear-right wheel, each of which is independent of each other.
In order to bring the turning state of the vehicle, provided with the brake conduit as described above, into the target state, a vehicle motion control device, for example, disclosed in the patent document 1 exerts braking force by brake fluid pressure on only the rear wheel at the inner side of the turning direction to thereby cause a yawing moment in the direction same as the turning direction, when the turning state of the vehicle is in the understeer state. On the other hand, this device exerts braking force by brake fluid pressure on only the front wheel at the outer side of the turning direction to thereby cause a yawing moment in the direction opposite to the turning direction, when the turning state of the vehicle is in the oversteer state. Specifically, this device exerts braking force by brake fluid pressure on only one wheel in one circuit of the above-mentioned dual circuit to which the front wheel at the outer side of the turning direction and the rear wheel at the inner side of the turning direction belong.
[Patent Document 1]
Japanese Unexamined Patent Application No. HEI10-16738
It has widely been known that the maximum road friction force (accordingly, the maximum braking force) that can be generated on a tire in the advancing direction of the tire decreases as the road friction force (i.e., cornering force) generated on the tire in the direction perpendicular to the advancing direction of the tire increases. Further, upon the turning of the vehicle, the cornering force generated on the tire in the direction perpendicular to the advancing direction of the tire generally increases with the increase in the centrifugal force exerted on the vehicle as the vehicle body speed increases. Accordingly, the maximum braking force that can be generated on the tire in the advancing direction of the tire upon the turning of the vehicle decreases as the vehicle body speed increases.
From the above, in case where the vehicle turns at a relatively high speed and its turning state becomes a relatively excessive understeer state or a relatively excessive oversteer state, for example, there may be the case where the magnitude of the braking force by the brake fluid pressure that should be exerted on one wheel of one circuit exceeds the maximum braking force that can be generated on the tire of the wheel in the disclosed device. In this case, the braking force by the brake fluid pressure exerted on the aforesaid one wheel is limited to the maximum braking force. As a result, a yawing moment having the expected magnitude is not caused on the vehicle, thereby entailing a problem that the understeer state or the oversteer state cannot surely be eliminated.
On the other hand, if the vehicle can effectively be decelerated (i.e., if the centrifugal force exerted on the vehicle can be reduced) in this case, the maximum braking force increases to thereby cause more greater yawing moment on the vehicle, so that the aforesaid problem can be solved.